Charles Townes:
It presupposes a targeted place, and it presupposes you know what the
attenuation is between you and them. That's not so easy, but one could make some
approximation to it. I think it can cause a little trouble. I don't think it's a
very serious problem for initial efforts, at least. Any further comments? Well,
I guess this makes it my turn.
One aspect I would like to bring up is the
following: I am primarily working in astronomy now. In astronomy, there is so
much interest and so much drive in just finding out things about stars and about
planets and so on, that I would certainly expect that within the next two
decades that we will have located the stars in our immediate vicinity which have
substantial planets. The kind of distances we are talking about are about a
hundred or a thousand light years. Presently, systems are being planned which
should be able to do that. They're tough, they're difficult; it's beyond what we
have done yet, but nevertheless I think they are practical. And so, we will have
located planets.
Now the question is; how much further can an
intelligent community go? Would they be able to look at the planets and say that
"well now, this planet has this kind of an atmosphere, and
temperatures" and so on. To what extent could they find out still more
information and thereby be very selective in deciding which planets they would
be interested in communicating with? I think there is a substantial possibility
there and that will progress regardless what SETI does. A lot of information
will come in over the next few decades; a great deal of information. That, too,
will change the problem for us, and may well have changed the problem for a more
advanced civilization because they will know a great deal more when they pick
out, let's say, the one star in a thousand that looks really interesting and
leave the rest of them alone.
The other point I make at this time is that
several people have talked about and asked about this "quantum noise".
I'm sure it's clear to some of you, but not everyone, that quantum noise is
necessary because of the uncertainty principle. The uncertainly principle
implies that you cannot measure both phase of a wave and the number of photons
in the wave accurately at the same time. This is why, if you use heterodyne
detection which is linear, it reproduces the phase of the wave to some extent,
at least. Whenever you heterodyne-detect, you have the possibility of measuring
phase and hence you automatically must introduce noise. You are introducing
noise in the heterodyne process itself. That gives you a noise, and typically
for a normal heterodyne detection, that's one quantum per unit bandwidth per
second. Which is a lot of noise as you get to high frequencies. In the radio
range, it's relatively negligible.
The way around that is to use photon
detection, where you simply let a photon generate an electron in solid such as a
photodiode, and you detect a quantum. You then know nothing at all about phase.
There is no need to have any uncertainty whether there is a quantum there or
not. This is why at the higher frequencies the quantum detection is enormously
better. You do not produce the noise in your detection the way that linear
detection or heterodyne detection would do. Hence there's no reason to have any
background noise. Thereby you can detect single photons, even have very wide
bandwidths if you can make the background sufficiently low. The background of a
distant star is pretty low and you can also blank out the star to a certain
extent. So, one can go a long distance in that direction. There is still the
quantum fluctuations in the strength of the signal which people have pointed
out. That's there, but simply for detection itself one can generate situations
and do work which would then allow almost no background at all. Heterodyne is a
great thing for radio astronomy, but it is very hazardous for the optical
region.
Barney Oliver:
That noise on detection is important too for some extent isn't it, Charlie? If
your expectation is much less than a photon per sampling time, or per
observation time, then that noise itself is high. You would say "no, there
is nothing there" when there really might have been!
Charles Townes:
If you were looking for something and you don't find it in one minute, and then
three minutes later you find something, then you say "OK, I've detected a
signal".
Barney Oliver:
In three centuries, it's different! [laughter]
Charles Townes:
Sure - even in three centuries in principle, if there is absolutely no
background then you'd say "Wait a minute! I've got something! I've got a
photon!". You can integrate for a long time and in principle you can
integrate over a very large bandwidth if the background really is sufficiently
low.
Let's go on to Fred. Would you like to say
something, and then we will have general comments and questions?